It is an extremely important matter to control a precision of amount of removed water in a hemodialysis, and various methods have heretofore been employed such as adjusting a pressure at the blood side and a pressure at the dialysate side by means of a dialyzer through an ultrafiltration.
However, the elimination of water in the hemodialysis is such that an amount of removed water is an extremely small amount when compared with a flowrate of the dialysate so that in order to perform a water elimination control of a high precision, for example, there is a method of controlling the amount of removed water upon finding an ultrafiltration ratio intermittently. But, in general, since the ultrafiltration ratio is affected by a multitude of variable elements such as differences of physical properties according to kinds of dialyzers, differences of compositions of the bloods to be dialyzed or fluctuations with respect to the passage of time, it has been difficult to control the designated amount of removed water with a high precision. Also, in the clinic of the dialysis, as proteins, fats and the like are contained in the liquid discharge from the dialyzer, there are problems of causing errors in the measurement due to an adhesion to paths and scaling structure, and in a method of controlling an amount of removed water by using a piston pump and the like provided with mobile components, there are problems of increasing mobile resistance and accelerating wear and tear which result in errors due to the passage of time.